Page:Reflections on the Motive Power of Heat.djvu/85

Rh has taught us nothing on this subject. It has only shown us that this caloric is developed in greater or less quantity by the compression of the elastic fluids.

This preliminary idea being established, let us imagine an elastic fluid, atmospheric air for example, shut up in a cylindrical vessel, abcd. (Fig. 1),

provided with a movable diaphragm or piston, cd. Let there be also two bodies, A and B, kept each at a constant temperature, that of A being higher than that of B. Let us picture to ourselves now the series of operations which are to be described:

(1) Contact of the body A with the air enclosed in the space abcd or with the wall of this space—a wall that we will suppose to transmit the caloric readily. The air becomes by such contact of the same temperature as the body A; cd is the actual position of the piston.

(2) The piston gradually rises and takes the position ef. The body A is all the time in